External photodetector cooling techniques

ABSTRACT

A technique is disclosed for improving the performance of photoemissive devices such as the photocathode of a photomultiplier tube by reducing thermal electron emission noise and thus increasing the signal to noise ratio. A thermoelectric cooler is positioned outside the evacuated envelope of the photoemissive device but in thermal communication with the device through a thermally conductive link arranged within and extending through the wall of the envelope. This conductor places the photoemissive device in direct heat exchange relationship with the external thermoelectric cooler while isolating the cooler from the remainder of the tube structure whereby the device is efficiently cooled without the need for cooling the remaining elements of the tube. This permits efficient cooling while at the same time avoiding the expense and bulk of conventional cryostatic coolers.

United States Patent [19] Ace [ 1 EXTERNAL PHOTODETECTOR COOLINGTECHNIQUES [75] Inventor: Ronald S. Ace, Greenbelt, Md.

[73] Assignee: Ace Sophisticates Incorporated,

Greenbelt, Md.

[22] Filed: Jan. 19, 1973 [2]] Appl. No.: 324,922

[52] US. Cl 313/39, 313/94, 60/3 [51] Int. Cl. H0lj 1/02 [58] Field ofSearch 313/39, 94

[56] References Cited UNITED STATES PATENTS 3,064,440 ll/l962 Waller..3l3/39 Primary Examiner-Herman Karl Saalbach Assistant Examiner-DarwinR. Hostetter Attorney, Agent, or Firm.lones and Lockwood June 4, 1974ance of photoemissive devices such as the photocathode of aphotomultiplier tube by reducing thermal electron emission noise andthus increasing the signal to noise ratio. A thermoelectric cooler ispositioned outside the evacuated envelope of the photoemissive devicebut in thermal communication with the device through a thermallyconductive link arranged within and extending through the wall of theenvelope. This conductor places the photoemissive device in direct heatexchange relationship with the external thermoelectric cooler whileisolating the cooler from the remainder of the tube structure wherebythe device is efficiently cooled without the need for cooling theremaining elements of the tube. This permits efficient cooling while atthe same time avoiding the expense and bulk of conventional cryostaticcoolers.

18 Claims, 6 Drawing Figures PATENTEDJUN 4 m4 sum 1 or 3 l EXTERNALPHOTODETECTOR COOLING TECHNIQUES BACKGROUND oF THE INVENTION The presentinvention relates, in general, to a method and apparatus for improvingthe signal to noise ratio of photoemissive devices, and moreparticularly to an improved apparatus for reducing the temperature ofthe photocathode in a photoemissive device such as a photomultipliertube, image intensifier, or the like, by placing in direct thermalcontact with the photocathode a thermoelectric cooler. The cooler is inheat exchange relationship with the photocathode through the wall of theevacuated envelope of the tube by way of a suitable thermally conductivelink.

It is presently common practice to cool photomultiplier tubes in orderto reduce their temperatures and thus decrease the amount of darkcurrent" which occurs by reason of thermal emission of electrons fromthe photocathode material. Such cooling would also be desirable forother photoemissive devices used for detecting small quantities oflight, but this has not been generally done sincethe methods andapparatus for cooling used in the prior art have been prohibitivelyexpensive, as well as being so bulky and awkward that they have falleninto the category of last resort devices to be avoided if at allpossible. Since the primary application to date has been limited to suchdevices, the present invention will be described in the context of aphotomultiplier tube; however, it will be understood that the presentinvention may be utilized in combination with other photoemissivedevices where the detection of light and its conversion to correspondingelectric signals to be accomplished, and it will be seen thatphotodetectors in general may be cooled by the method and apparatus ofthe present invention without departing from the scope thereof.

Photoemissive detectors, which are devices used to detect and measurevery small quantities of light, or photons, are sensitive to, and arecapable of measuring, light of various wavelengths, ranging from beyondthe X-ray region through the visible wavelengths and into the infraredregions, depending upon the active materials used on the photoemissivesurface. All photosensitive devices take advantage of the fact thatunder certain conditions a given photocathode material will emit one ormore free electrons per incident light photon. By means of thisphotoelectric effect, the light photon is transduced into an equivalentnumber of photoelectronswhich constitute an equivalent electricalsignal. In a photomultiplier tube. the emitted photoelectrons undergohigh multiplication, or amplification, in a series of dynode stages,each of which releases secondary electrons upon impact by a primaryelectron. By appropriate selection of the dynode material, numerouselectrons are emitted by each impact from a primary electron, wherebythe small number of photoelectrons emitted by the photocathode ismultiplied. This increased number of secondary electrons is ultimatelyreceived at the output anode of the tube to provide an electrical pulse,or count, whereby the light received by the photomultiplier tubeproduces an analogous output signal.

Although photoemissive devices of this type are remarkably sensitive, attheir lower limits, they are found to exhibit electrical noise which isindistinguishable from photon-produced signals, and such noise preventsLII generated noise is indistinguishable from the desired photoelectroncurrent, this noise cannot be filtered out or otherwise removed from thedetector output signal. This is because photocathode noise andphotoelectrons both occur naturally as single electron events, with thenoise producing a dark current which renders the detection of weak lightsignals nearly impossible. The present invention recognizes the factthat if very minute quantities of light are to be measured, it isessential that thermal emission from the photocathode of aphotosensitive detector be eliminated and that this can be accomplishedby maintaining a reduced temperature on the photocathode.

The prior art has recognized that spurious emissions,-

or dark counts, can be reduced in photoemissive devices by utilizingvarious techniques. But after all of the known electronic and opticaltechniques have been exhausted, designers usually resort to cooling theentire device in order to achieve the very best signal to noise ratio.The advantages of cooling photoemissive devices can, in some cases, bevery significant. Under some conditions, photocathode noise can bereduced by factors greater than 10,000 and because this advantage is soworthwhile, many device users purchase cooled housings, known ascryostats, at prices which are often much greater than the tube that itis intended to cool. Although a cryostat is capable of reducing aphotoemissive device to a temperature sufficiently low to produce asignificant reduction in the photocathode noise events, the use ofcryostats has not generally been a satisfactory solution to the problem.Not only is a cryostat costly, but its size and weight nearly alwaysdwarfs that of the photodetector and size and weight are importantconsiderations in many applications of these devices. Thus, the totalsize and weight of present day photoemissive detectors and their coolingsystems is usually a prohibitive deterrent to designers.

Cryostats suffer additional disadvantages in that they are relativelyinefficient, and require periodic maintenance of the coolingrefrigerants, which may be dry ice, liquid nitrogen, or the like. Wherethe cryostat is thermoelectrically powered, it usually will requirewater or forced air cooling of the thermoelectric pile and and this initself produces problems in that there must be continuous maintenance ofthe circulating pumps and fans, water filters, and the like. Thus, allof these cooling systems requre constant attention. In addition to theforegoing deterrents to the use of photoemissive devices and theircoolers is the fact that cryostats, although presumably sealed andoperating with very dry air, are often plagued with the problem ofcondensation, forcing the user periodically to open the device fordefogging and/or defrosting the system, thus possibly exposing thephotosensitive device to bright lights which can be harmful to thephotoemissive surfaces. Still another deterrent to the use of cryostatsis the fact that they usually do not incorporate provisions to prevent atoo rapid temperature change in the detector device. Since these devicesusually are enclosed in metal and glass envelopes, a too rapid change intemperature can and often does fracture the envelope near the pins ofthe tube or at other glass to metal seals. The

uneven thermal shock and resultant stress from such cooling hasdestroyed many valuable detectors inadvertently in attempts to cool thewhole tube too fast. This problem is particularly acute where themultiplier tube must be exchanged periodically, or where the tubes arecooled, allowed to return to room temperature, and subsequentlyrecooled. To avoid thermal shock during this procedure, it is necessaryto reduce the temperature gradually and it is often inconvenient to haveto wait for many hours while the system gradually reaches thermalequilibrium; however, it is equally inconvenient to have to leave thecryostat in operation at all times so that the tube can be usedoccasionally. Because of these major difficulties with the cooling ofphotoemissive devices, it has long been evident that improved techniquesfor cooling such devices were needed.

A step forward in the direction of improving the cooling ofphotoelectric devices was provided by the invention disclosed incopending application Ser. No. 279,922, filed on Aug. l 1, i972, andentitled Internal Cooling for Photodetectors." That applicationrecognized that the only portion of a photosensitive device that neededcooling was the photocathode, and that a cryostat accomplishes thiscooling with a shotgun approach which is inefficient since it coolsportions of the devices which do not need cooling. Accordingly, theapplication was directed to a method of cooling solely the photoemissivestructure in an efficient manner, with a lightweight, inexpensivethermoelectric cooler located within the evacuated envelope of thephotoemissive device. However, although that prior application producednumerous advantages over the prior methods of cooling photoemissivedevices, several difficulties were encountered, caused by the fact thatthe processing of ultrahigh vacuum devices such as photoemissivedetectors usually require high bakeout temperatures; in somemanufacturing processes, the bakeout temperatures may exceed 600C.Unfortunately, some of the most desirable thermoelectric coolermaterials such as Bismuth telluride semiconductors and alloys cannoteasily be raised to such a demanding temperature, for they either meltbefore reaching such a temperature, or the temperature createsintolerable vapor pressures. in order to produce photodetectors usinghigh temperature curing techniques, thermoelectric devices such as LeadTelluride semiconductors and alloys capable of withstanding suchtemperatures have to be used if they are to be incorporated within thetube envelope. However, such higher temperature devices have lowerefficiencies and are less desirable. Accordingly, if the best possiblethermoelectric efficiencies are to be utilized in cooling photoemissivedevices, other techniques must be used.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to overcome the difficulties inherent in prior art coolingsystems for photoemissive devices by providing an inexpensive, easy touse cooling system which completely eliminates the bulk and expense ofprior art cryostats by locating a thermoelectric cooler in heat exchangerelationship with only the photoemissive surface of the device.

It is a further object of the invention to provide means for coolingphotoemissive devices located within sealed, high vacuum envelopesquickly and efficiently while at the same time reducing the cost andtime delays inherent in prior devices.

it is a further object of the present invention to provide means forreducing the temperature of a photoemissive surface by close thermalcontact between the surface and a thermoelectric cooling element.

It is another object of the present invention to provide efficient,inexpensive, cooling of the photocathode area of a photoemissive deviceto enhance the signal to noise ratio of the electrical signals producedby the device in response to impinging light by providing means forrapid and accurate temperature control of the photocathode. I

It is another object of the present invention to provide cooling meansfor a photodetector device which has low power consumption and increasedcooling efficiency due to the provision of means for cooling only thephotoemissive surface of the photodetector.

Briefly, the present invention accomplishes the foregoing and otherobjects by positioning a thermoelectric cooling element of the typeexhibiting the Peltier effect adjacent the exterior surface of theenvelope enclosing a photoemissive detector device. The thermoelectricelement is secured in intimate heat transfer relationship with thephotoemissive surface through the medium of a thermally conductivematerial such as tungstan, aluminum, various other metals, ceramics, andeven some glasses, in the wall of the envelope and within the envelope,which material forms a thermally conductive link between the cooler andthe element to be cooled.

Because of the large variety of photosensitive devices, such asphotomultipliers and image intensifiers, and the more common imagingpick-up devices such as image orthicons, silicon intensifier tubes, andsecondary electron conduction tubes used in the television industry, andbecuase of the variety of configurations which they take, the particularmanner in which the cooling surface of ther thermoelectric cooler issecured to the exterior surface of the envelope, and the particulararrangement by which the photoemissive surface is placed in heatexchange relationship with the thermoelectric element will vary.However, the primary advantage of any present arrangement is that thethermoelectric cooling elements will be positioned on the outside of theenvelope of the photosensitive device after heat tratment of theenvelope, thus eliminating any possible contamination inside theenvelope and preventing possible destruction of the thermoelectriccooling apparatus. Thus, the very best thermoelectric cooling materials,such as Bismuth Telluride and similar alloy semiconductors exhibitingthe Peltier effect, may be chosen as coolers without concern forproblems such as the temperature limitations of such materials duringmanufacturing processes.

The present invention differs from other external cooling arrangementsby its'ability efficiently to cool the photocathode area only, withoutcooling the entire photodetector envelope. This arrangement permits highefficiency, since there is a relatively low heat load presented to thecooler, the heat load being a function of the relatively small cooledarea and the associated thermally insulating properties of the vacuumwithin the envelope in which the photoemissive surface is located.

In a preferred form of the invention, the envelope for thephotosensitive device is formed with a heat transfer segment such as ametal band, plate or disc, which may be secured in the envelope wall byconventional metal glass seals. This heat transfer segment of theenvelope is so located as to be as near as practicable to thephotoemissive cathode located within the tube, and a thermal link isprovided to place the photocathode in direct thermal contact with thesegment. For example, the photocathode may be physically secured withinthe envelope by means of a support structure which is thermallyconductive but electrically insulating, the support structure beingsecured to a heat conductive band in the envelope wall. Alternatively, athermally conductive link may be positioned in the tube to contact thephotocathode, which may be supported by other elements within the tube,the thermally conductive link merely serving to transfer heat from thephotocathode to the exterior of the envelope. The metal plate or bandsecured in the envelope wall and the thermally conductive linkcontacting the photocathode are of materials which are capable ofwithstanding the high curing temperatures required in the manufacture ofsuch devices.

Upon completion by a conventional manufacturing process of aphotoemissive device having an envelope incorporating a heat transfersegment, the thermoelectric cooling element may be secured to theexterior surface of the heat transfer segment, with the cold surface ofthe thermoelectric device in contact with the envelope segment and thehot surface of the device in contact with the suitable heat sink outsidethe tube. Because of the low heat load of such a device, the heat sinkneed not be water cooled, and is formed in accordance with known heatexchange techniques to provide the required cooling surface.

In general keeping with good high-vacuum photodetector designtechniques, high vapor pressure materials must be kept out of thedetector envelope in order to reduce the possibility of condensatesbeing deposited on sensitive surfaces such as the electron multipler orphotocathode surfaces. Such deposits are particularly a problem if thephotocathode structure is cooler than the remainder of the tubeComponents, for the condensates tend to collect on such a coolersurface. Fortunately, excellent construction materials forphotodetectors do exist that provide a very low vapor pressure level formost of the tube components. Materials such a Berilium orBerilium-Copper, for example, may be used for the electron multiplierstructure, and thus reduce this problem. No particular precautions needto be taken with respect to the vapor pressure of the material used forthe photocathode, on the other hand, since the cathode is the maincomponent being cooled and thus encourages its own vapor, if any, tocondense on itself.

Since the cooled portion of the tube is limited to the photocathode andto a relatively small portion of the envelope surface at the metal bandheat exchange area,

fogging and frost build-upon the optical and electrical surfaces is nota problem with the present invention, because these surfaces are nevercooled in the presence of moist air. The photocathode is locatedentirely within the vacuum envelope, and the optical surface of the tubeis insulated from the photocathode by the vacuum of the tube. Because ofthe low heat load, cool down time to equilibrium is significantlyreduced compared to that of presently available systems, and the problemof thermal fracturing is greatly reduced. With the thermoelectricdevice, the cooling rate can be fully and easily controlled, and byselection of appropriate BRIEF DESCRIPTION OF THE DRAWINGS The foregoingand additional objects, features and advantages of the present inventionwill be apparent to those skilled in the art from the following detaileddescription of preferred embodiments thereof, taken with theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view of a conventional end on" photomultipliertube utilizing a thermoelectric cooler in accordance with the presentinvention;

FIG. 2 is a cross-sectional view of the tube of FIG. 1, taken along'line22 thereof;

FIG. 3 is a diagrammatic sectional view of a conventional side on"photomultiplier tube utilizing the external thermoelectric coolingsystem of the present invention;

FIG. 4 is a cross-sectional view of the tube of FIG. 3, taken along line4-4 thereof;

FIG. 5 is a partial sectional diagrammatic view of an image pick-up tubeutilzing the thermoelectric cooling techniques of the present invention;and

FIG. 6 is a partial sectional diagrammatic view of another version of aconventional photomultiplier tube utilizing the external thermoelectriccooling system of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT In order to cool efficiently thephotocathode area of a photosensitive device, a suitable thermalconduction path must be established between the photocathode and thecold surface of a thermoelectric cooler. In accordance with thisinvention the cooler is external to the envelope of the photosensitivedevice, but in initimate heat exchange relation with the photocathodethereof. Because of the extremely wide variety of photoemissive devicespresently available, the present invention is illustrated with a varietyof representative devices which may be utilized in combination with thisinvention. Referring more particularly to the drawings, there isillustrated in FIG. I a photomultiplier tube generally indicated at 10which may be of the conventional end-on type wherein the photocathode isa thin semitransparent semiconducting film such as CS Sb, (Ca)- Na KSb,(CS)Na KSb, or the like, deposited on the glass window through whichlight enters the device. The photomultiplier tube It} includes anevacuated envelope lll having a sidewall I2, base 14 and a face plate16, the face plate being of an optically transparent material such asglass and the sidewall and base perferably being of glass or otherconventional materials of low thermal conductivity. Mounted within anevacuated chamber 18 defined by the tube envelope It is an opticallytransparent substrate 20 which is mounted so as to be parallel to faceplate It: and closely spaced there from to receive light in the form ofphotons 22 which is to be detected, and which passes through the faceplate. A thin film coating 24 of photoemissive semiconducting materialis supported by the substrate and forms a photocathode, whereby theentering photon 22, upon passing through substrate 20, interacts withthe photoemissive material to produce a free electron, diagrammaticallyindicated at 26. The photoelectron, after emission, is caused to followa substantially straightline path 28 to the multiplier stage of thetube. in the illustration of this figure, the miltiplier isdiagrammatically shown to be that of a commercially available multiplierdevice known as a Channeltron, generally indicated at 30. TheChanneltron, which is manufactured by the Bendix Corporation, isfunctionally similar to other known photomultiplier devices, although itis of smaller size and by reason of its configuration is relativelyimmune to stray magnetic fields.

As is known in the photomultiplier art, the photoelectron 26 strikes thesurface of the Channeltron device 30 at its input end and causes therelease of secondary electrons. Each emitted electron is then drawn to asecond portion of the Channeltron device, in the manner of electronsstriking succeeding dynodes of a conventional photomultiplier tube,thereby producing the output of the Channeltron a corresponding packetof secondarily emitted electrons, which are collected on an output anode32, and apeear on an output pin 34 for the tube as a small electricalpulse, or single count, which is analogous to the input photon. Powersupply leads 36 and 38 are provided for the Channeltron device 30 toprovide the required biasing, and these leads also extend through thebase 14 of the tube in the form of pins for easy connection to externalcircuitry, in known manner. As electrical lead 40 is provided for thephotocathode, this lead also extending through the base 14 of the tubein the form of a pin.

The substrate 20 which carries the photocathode 24 may be secured withinthe tube envelope by means of an annular support base 42 which is formedwith an interior peripheral channel 44 to receive the edge of thedisc-shaped substrate. The support base 42 firmly clamps the edge of thesubstrate to provide not only strong mechanical support but a goodthermal contact so that there will be adequate heat transfer between thesubstrate and the base support element. Support base 42 is constructedof a material which has good thermal conduction characteristics, and itsouter circumferential surface is secured to the sidewall of the envelopeto hold the substrate and photocathode in the proper location within thetube envelope and to serve as a thermal link between the photocathodeand the exterior of the envelope. This unit may be secured in place by apressure fit, by soldering, or by any other convenient means in such asway as to provide a good thermal contact between the annular supportbase and the sidewall 12 ofthe tube. Since in the illustrated embodimentthe power is supplied to the cathode 24 by way of lead 40 connected tothe support base 42, it is apparent that the base must only bythermally, but electrically, conductive in this case.

As has been indicated, there are numerous sources of noise within aphotomultiplier tube of the type illustrated in FIG. 1, but most sourcesproduce signals which can be identified and eliminated eitherelectronically or optically from the resultant output signal. However,spurious electrons emitted by the photocathode in the absence of anincident light photon cannot be distinguished from a photoelectronemitted in response to a photon, and both will produce secondaryemission in the Channeltron device which will produce a resultant outputsignal. Accordingly, if the operational characteristics of the tube areto be improved, the noise signal generated by such spurious electronsmust be eliminated. This is accomplished in the present invention bymeans of a thermoelectric cooling system generally indicated at 46,which is located adjacent the exterior surface of a selected heattransfer segment 48 of the sidewall 12 of the tube and is in intimateheat exchange relationship with that portion of the envelope wall towhich the photocathode is secured by the support base 42, the supportbase and the wall segment thus forming a thermal link between thephotocathode and the cooler.

The thermoelectric cooling system 46 incorporates a pair ofthermoelectric elements 49 and 50 which are generally annular in shapeand are secured to a common internal circumferential cold junctionsurface defined by a ring 51 and to a common external circumferentialhot junction, defined by a ring 52. The thermoelectric elements 49 and50 are each made up of one or more couples-each of which may comprise apair of a great variety of materials which exhibit the Peltier effect,such as Bismuth Telluride and Lead Telluride, all of which have a heatpumping ability and thus are capable of producing a temperaturedifferential between two spaced surfaces when connected electrically inseries, upon application of an appropriate electrical current. Peltiereffect devices may be constructed in virtually any geometricalconfiguration, and thus the exact size and shape of the device utilizedwith the photomultiplier tube 10 will depend upon the me chanicalconstruction of the tube, of the photocathode, of the support elementsfor the photocathode, and of the mechanical arrangements of elements.

An example of suitable thermoelectric cooling elements for applicationin the device of the present invention may be found in the publicationof the Thermoelectrics Department of the Borg-Warner Corporation, DesPlaines, lll. This publication is entitled The Where and The Why ofThermoelectric Cooling" by G. F. Boesen, C. J. Phetteplace, and LJ.Ybarrondl, and was copyrighted in l967. Another publication describingsuitable thermoelectric devices is the Thermoelectric Manual" publishedby the Cambridge Thermonic Corporation, 445, Concord Ave. Cambridge,Mass.

As noted in the Borg-Warner publication identified above, athermoelectric cooler utilizes semiconductor materials with dissimilarcharacteristics connected electrically in series and thermally inparallel, so that two junctions are created. The semiconductor materialsare N and P-type and are so named because with they have more electronsthan necessary to complete a perfect molecular lattice structure(N-type) or not enough electrons to complete a lattice structure (P-type). The extra electrons in the N-type and the holes left in theP-type material are called carriers, and they are the agents that movethe heat energy from the cold to the hot junction. Heat absorbed at thecold junction is pumped to the hot junction at a rate proportional tothe carrier current passing through the circuit and the number ofcouples. Good thermoelectric semiconductor materials such as BismuthTelluride impede conventional heat conduction from hot to cold areas,yet provide an easy flow for the carriers. in addition, these materialshavecarriers with acapacity for carrying more heat.'ln practical use, aplurality of couples are combined in a module which can be tailored tothe exact requirements of the user. These modules come in a greatvariety of sizes, shapes, operating currents, operating voltages, numberof couples, and ranges of heat pumping levels, although the presenttrend is toward a larger number of couples operating a low current.

In the illustrated embodiment of FIGS. 1 and 2 the thermoelectricelements 49 and 50 each consist of a plurality of thermocouples arrangedin series to form an annular ring adapted'to fit around the envelope llof the photodetector 10. As shown in FIG. l,the rings are arranged inparallel, coaxial, side-by-side relationship and are in thermal contactwith cold and hot junction rings 51 and.52.

As shown in FIG; 2, the thermoelectric element 49 is made up of a seriesof couples which extend in an annular ring" about the axis of the tube.Thus, for example, a first couple consists of a block of P-type material53 connected at its upper end of an electrically conductive plate 54which is connected in turn to an electrical lead 55. The lower end'ofblock 53 is connected to an electrically conductive-plate 56 whichinterconnects the block with the bottom of anN-type semiconductive block57. Plate 56 thus forms the junction between the P and N-type materialsof this couple. The upper end of the semiconductive block 57 isconnected by way of a conductiveplate 58 to the upper end of block 58which is the P-type material of a second couple in the series. The lowerend of block 58 is connected by a conductive plate 59 to the lowerportion of its corresponding N-type semiconductive block 60 to completethe second couple. In similar manner, the upper end of block 60 isconnected to the next couple in the series by way of electricallyconductive plate 63; Successive couples are similarly connected to eachother in series around the thermoelectric element 49, with the Ntypematerial 64 forming the last portion of the last couple in the series.The upper end of block 64 is connected by way of conductive plate 65 toan electrical lead 66. A direct source (not shown) having its positiveterminal connected to line 66 and its negative terminal connected toline 55 causes a current to flow in series through the plurality ofcouples, creating a pumping action which produces a heat flow away fromthe conductive plates 56, 59, etc. and toward the conductive plates 54,58, 63, etc. whereby heat is pumped from the cold junction ring 51 tothe hot junction ring 52 in known manner.

The rings 51 and 52 are electrically non-conductive, but are of amaterial having a good thermal conductivity to which the thermocouplescan be secured by any convenient means, such as by soldering, epoxybonding, or the like. The inner surface of cold junction ring 51 is inintimate heat conductive contact with the heat transfer segment 48 ofthe sidewall 12 of the tube,

while the outer surface of the hot junction 52 is similarly in intimateheat conductive contact with a surcurrent flow which is the reverse ofthat shown in FIG. 2 for element 49. That is, if the current is flowingfrom the positive source terminal through element 49 to the negativesource terminal in a counter-clockwise direction to produce the desiredheat pumping, then element 50'preferably is constructed to produce thedesired direction of heat pumping with a clockwise flow of current asviewed in FIG. 2. Since photosensitive devices are, in general,Sensitive to magnetic fields, the provision of two parallel butoppositely sensed thermopile loops 49 and 50 as illustrated in FIG. 1 isparticularly advantageous. The magnetic field generated by the netclockwise current flow through one series of thermocouples tends to becancelled by the magnetic field generated'by the net counterclockwisecurrent in the oppositely sensed parallel loop of thermocouples, all ofthe thermocouples being in series electrical connection. Thisgeometrical configuration reduces the effect of the thermoelectriccooling current on the electrical operation of the tube. In addition,suitable shielding, the use of coaxial power lines, and the use of lowcurrent, high voltage series arrays in the thermopiles further reducethe problem of magnetic interference. Since the Channeltron deviceillustrated in FIG. 1 is particularly immune to stray magnetic fields.the use of the foregoing techniques with such a device insures thatthere will be virtually no problem of magnetic interference.

It .will also be noted that in some applications the production of amagnetic field by the thermopile loops illustrated in FIG. l can beuseful in controlling the flow of the photoelectrons 26. For example, amagnetically focused photomultiplier tube could utilize the magneticfield of the thermopile to limit an electron imaging aperture, with acorresponding reduction in the size of the effective photocathode area.This procedure results in further reduction of photocathode noise.

The thermoelectric elements 49 and 50 are firmly secured to the cold andhot junction rings SI and 52 which preferably are of a thin flexiblematerial to enable the cooling elemcnts to be flexed for positioningabout the sidewalls of the envelope ll. Alternately, the cooling devicemay be formed in semicircular pieces which may be placed about theenvelope and secured by a suitable clamp or by soldering the abuttingends of the rings together. Alternatively, or in addition, the innerjunction ring 51 may. be soldered or epoxy bonded to the tube segment 48to insure a good conductive relationship and the outer ring 52 maysimilarly be soldered or otherwise bonded to the heat sink.

As illustrated in FIG. 1, the envelope may include an annular depressionor channel 68 formed in the side wall 12. The interior surface of thisannular channel may provide a supporting surface for the photocathodesubstrate 20 and its support base 42, and thus may define the segment ofthe envelope where the thermoelectric device is to be located. It willbe evident that the sidewall 12 maybe made cylindrical throughout itslength, with the thermoelectric device fitting around the outside, butthe illustrated construction is preferred since it enables the envelopesidewall to provide a protective housing for the thermoelectricelements. As has been indicated, the thermoelectric elements are.secured in place'around the tube sidewall by any suitable means, andthus may be clamped, soldered together or to the sidewall, or otherwisefirmly fastened in such a manner as to provide a good thermalconnection, with or without the use of thermal compounds such'as silicongrease or the like, with the sidewall. The heat transfer segment 48 ofthe tube sidewall may form the bottom of channel 68, and may bepositioned there by suitable metal to glass seals in accordance withmethods known in the art.

As is illustrated in FIGS. 1 and 2, the thermoelectric cooling elements49 and 50 secured within the channel 68 are surrounded by a thermallyinsulating foam 70 which fills the voids around the thermoelectric loopsand between the semiconducting blocks of each of the thermocouples inorder to increase cooling efficiency.

Although a highly thermally conductive path of metal, ceramic, or thelike is usually desirable between thethermoelectric cooler and thecathode structure, a relatively poor path can still be used where alarger temperature differential, between the cold junction and thephotocathode, with its consequent reduction in efficiency, producesacceptable operational characteristics. Thus, in some cases, it may besatisfactory to utilize the glass wall of a glass envelope, thusobviating the need for multiple glass to metal seals and therebysimplifying the device and reducing its cost. However, since the thermalemissions which generate noise signals in a photomultiplier tube occureven at temperatures well below room temperature, the cooling apparatusmust operate to reduce considerably the temperature of the photocathodein order to minimize this source of noise. A typical temperature forphotocathode operation is -25C., although some may be operated attemperatures as low as 200C. It is evident that where a photocathodematerial need not be reduced in temperature to the lowest ranges, aglass wall may provide a satisfactory degree of thermal conductivity.However, where the material of the thermal link is the same as that ofthe envelope, there will be some loss of efficiency due to the tendencyof the cooler to also cool the envelope. This problem together withother factors A such as the relative cost and the temperatures involvedmust be taken into consideration when determining whether to utilize anexisting glass envelope construction or to modify the photoemissivedevice to accomodate a metal, ceramic, or similar material in a sectionof the envelope wall for maximum thermal conductivlty.

Turning now to P16. 3, there is illustrated another embodiment of thepresent invention, wherein a thermoelectric cooler is applied to aconventional side-on version of the photomultiplier tube. Theconventional photomultiplier tube generally indicated at .80 is providedwith a base 82 and glass envelope 84 in which is mounted theconventional electronmultiplier array 86.

In this figure, the multiplier array is illustrated as including aphotocathode 88, which may again be a thin film of photoemissivematerial supported on a suitable substrate (not shown), mechanicallysupported in a suitable manner within the tube envelope. The substratemay be of metal to provide a uniform distribution of the support voltageto the photocathode material. Again, suitable supply voltages aresupplied to the tube elements by way of a plurality of pins 90.

A photon of light, indicated diagrammatically at 92, enters thephotomultiplier tube 80 through the glass envelope 84, passes through agrid 94, and impinges on the surface of the photocathode 88. Thephotoemissive surface emits a photoelectron, diagrammatically illusnode(not shown) and so on through the dynode array.

Electrons emitted by the last unit in the array are attracted to ananode to produce an output which corresponds to the input photon.

The cooling of the photocathode in the illustrated embodiment iseffected by means of a thermally conductive connecter element or link102, which is secured in intimate heat conductive relationship with thesubstrate on which the photocathode is mounted and spans the thermallyisulating vacuum between the photocathode and the tube envelope. Thisthermal link 102 preferably is electrically insulative and serves toprovide a heat conductive path between the photocathode and the cap 104of the tube envelope. This cap or heat transfer disc may be of glass ormetal, depending upon the heat transfer requirements of the device.Mounted on top of cap 104 is a generally disc-shaped thermoelectriccooler unit 106 which iscomprised of a pair of spaced parallel plates108 and 110 made up of metalized thermally conductive but electricallyinsulating material such as Alumina or Berylium Oxide, these platesserving as the cold and hot junctions respectively of the thermoelectricelement. Mounted between the hot and cold plates are a plurality ofconventional thermoelectric cooling units 112 through 117, which aresuitably energized, as by a pair of electrical leads 118 and 120 to pumpheat from the cold plate 108 to the hot plate 110.

The Peltier effect devices 112 through 117 are surrounded by a thermallyinsulating material 122 which may be a foam insulator or the like, toimprove the efficiency of the device and prevent condensation on thecool elements. In intimate contact with the hot surface 110 is a heatsink 124 which may be in the form of an inverted cup fitting over thetop of the tube and spaced from it at the sidewalls by spacers such asthose indicated at 126 and 128. The heat sink may be provided withcooling fins 130. and is designed to dissipate the heat from thephotocathode by way of the heat conductive thermal link 102, the coldplate 108, the Peltier effect devices 112 through 117, and the hot plate110. This heat sink includes an opening 132 aligned with thephotocathode 88 and grid 94 to admit light into the interior of thetube.

As illustrated in FIG. 4, which is a top view of a modified form of thephotomultiplier of FIG. 3 with the cooling fins'130, the top of the heatsink 124, and the hot plate 110 cut away, the thermoelectric cooler isseen to comprise a pair of concentric electrically conductive rings 134and 136 between which are connected the several thermocouples 112through 117. As illustrated in F168. 3 and 4, each thermocouple,112 forexample, includes a block of P-type semiconductive material 138 and ablock of N-type semiconductive material 140 joined at the bottomsurfaces by an electrically conductive plate 142.

A current applied to line 120 is fed by way of ring 136 through theN-type material, plate 142, and 'P-type material 138 to ring 134, thiscurrent flow providing a heat pumping action to cool the plate 142, andthus the cold junction plate 108, transferring the heat by way of rings134 and 136 to. the electrically insulating but thermally conductive hotjunction plate 110. In similar manner, each of the other thermocouples113 through 117 pump heat away from plate 108 toward plate 110 to effectcooling of the upper end 104 of the envelope and thus of the thermalconductor 102, whereby the photocathode of the tube is reduced intemperature. It will be noted that the current flow through thethermocouples of this arrangement produces a very limited magnetic fiedwhich has a minimal effect on the operation of the photomultiplier tube.Other geometrical arrangements and locations of the thermoelectriccooling elements and their corresponding heat transfer elements for usewith the side-on type of photomultiplier or other photoemissive deviceswill be apparent.

This tube as indicated at 160, is generally similar to that of theconventional end-on photomultiplier tube illustrated in FIG. 1, exceptthat an optical image is to cused on the photoemissive cathode 162 fromwhich it is translated into an equivalent electron image. This, anobject 164 is focused by means of suitable optics 166 onto thephotocathode 162 which is supported by an optically transparentsubstrate 168, the image passing through the optically transparent faceplate 170 of the tube'160 before striking the cathode. The electronimage 172 is accelerated in known manner to a target array, orintensifier stage, l74, and is maintained in an unscrambled conditionduring this transfer by means of an external field magnet 176. The imagemay be stored, intensified on target 174 or read out in the usual way,as by electron beam scanning, or the like.

Such image pick-up tubes are very sensitive devices which are limited intheir ability to detect light primarily by the quantum efficiency of thephotocathode material and by the level of electronic noise. However,photocathode cooling of image tubes can reduce cathode thermal noise bysignificant amounts in the same manner as previously described withrespect to photomultiplier tubes, and with similar efficiencies. This isaccomplished by providing a channel portion 178 in the tube envelope andby mounting the-photocathode in thermal contact with the base 180 of thechannel through the use of an annular support element 182 which servesas a thermal link. The'thermoelectric cooling device 184 is mountedwithin the channel 178 and incorporated a pair of spaced annularthermoelectric elements or thermopiles 186 and 188, secured betweeninner and outer circumferential rings 190 and 192 forming the cold andhot junctions,, respectively, of the thermoelectric device. Forconvenience in assembly, the thermoelectric device may be constructed intwo halves in the manner described with respect to the device of FIG. 1,and may be clamped or otherwise secured around the tube envelope in themanner described hereinabove. Secured to the outer cooling ring 192 isan annular cup-shaped heat sink 194 which fits inside the field magnetin order to contact the cold junction of the thermoelectric device andwhich extends outside the field magnet in order to provide sufficientcontact with the ambient air to produce the required amount of cooling.

As before, where the thermal conductivity of a glass envelope issufficient, the base 180 of channel 178 may 7 be-of glass; however, itispreferred that a band of metal be secured in the envelope in accordancewith wellknown techniques in order to provide maximum heat transfer fromthe photocathode to the heat sink 1-94. With this metal bandconstruction, the envelope does not provide the electrical insulationbetween the photocathode and the cooler, and the element must beelectrically insulating.

The final embodiment to be illustrated is a modified form of the end-ontype of photomultiplier tube shown in FIG. 1. This modifiedform isillustrated in FIG. 6, and utilizes the conventional gallium arsenidephotoemissive crystal, which replaces the semitransparent thin filmphotocathode illustrated in the prior device. As shown in FIG. 6, thegallium arsenide crystal 200 is supported by means of spring loadedbrackets 202 and 204 on a support block 206 which is in turn secured byany suitable means such as spot welding within a photomultiplier tubeenvelope having a sidewall 208. Forming the end of the tube is a faceplate 210 which is of an optically transparent material such as glassadapted to admit light represented by photon 212. This light enters thetube and strikes the surface of the gallium arsenide crystal 200,causing the crystal to emit an electron 214 which is then directed to aseries of dynode multiplier stages (not shown) in a commercially knownmanner.

The support block 206 for the crystal may be electrically conductive toprovide energization for the crystal via connector 227. ln thisinstance, the heat transfer segment may be comprised of a thin,metallic, flexible disc 218 similar to an aquadag connector, secured ina circular opening the sidewall 208 by means of conventional metal toglass seal. The block 206, and the metal diaphram 218 form a thermallink between crystal 200 and the electrically insulating but thermallyconducting cold junction 220 of a thermoelectric cooling module 222,which may take the form of one of the cooling modules illustratedhereinabove. The module has a hot junction 224 which is secured by anysuitable means as previously noted to a heat sink 226. The embodimentillustrated in this figure has the advantage not only of providing meansfor effecting cooling of a gallium arsenide photocathode, but alsoillustrates a method for providing a thermal link through the sidewallof a tube with a'minimum effect on the integrity of the tube structure.As before, a suitable foam insulation, 216, or the like, is place in andaround the thermoelectric module in order to increase efficiency.

Although the present invention has been illustrated in terms ofpreferred embodiments thereof, it will be apparent that numerousvariations may be made not only in the physical arrangement of thedevice, but in he materials used. Thuus, there has been shown anefficient, economical, compact, lightweight, convenient and reliablecooling system for general applicability to photoemissive opticaldetectors and although numerous variations and modifications will beapparent to those of ordinary skill in the art, it is desired that theforegoing descriptions not be considered limiting but only exemplary ofthe present invention, and that the true spirit and scope thereof belimited only by the following claims.

I l claim:

l. in a photoemissive device, means for reducing the thermal emission ofelectrons, comprising an evacuated envelope;

a photoemissive surface within said envelope;

a thermal link between said surface and a portion of said envelope; and

a thermoelectric cooling element outside said envelope and in heatexchange relationship with said portion of said envelope, said coolingelement being energizable to pump heat from said thermal link and thusfrom said surface in order to cool said surface.

2. The photoemissive device of claim 1, wherein said thermal linkcomprises a highly thermally conductive element in intimate heatexchange relationship with said photoemissive surface and with saidenvelope.

3. The photoemissive device of claim 2, wherein at least said portion ofsaid envelope is of a highly thermally conductive material.

4. The photoemissive device of claim 3, wherein said envelope isprimarily of glass, said portion of said envelope being metal.

5. The photoemissive device of claim 4, wherein said photoemissivesurface comprises a photocathode.

6. The photoemissive device of claim 5, wherein said thermal linksupport said photocathode within said envelope.

7. The photoemissive device of claim 5, wherein said thermoelectriccooling element comprises a Peltier effect device and means for applyingelectric power to said device for energization thereof.

8. The photoemissive device of claim 7, wherein said thermal link is anelectrical insulator.

9. The photoemissive device of claim 8, further including a heat sink inthermal contact with said cooling element for dissipating heat pumpedfrom said photocathode.

10. The photoemissive device of claim 9, wherein said cooling elemnt isin heat exchange relationship with only said portion of said envelope,whereby only said thermal link and said photocathode are cooled.

11. The photoemissive device of claim 10 wherein said portion of saidenvelope comprises thermally isolated section forming a portion of thewall of said envelope, said thermal link providing a heat conductivepath between said photocathode and said wall section, and the vacuumwithin said envelope thermally insulating said photocathode and saidthermal link from the remainder of said envelope. whereby the remainderof said envelope is not cooled by said cooled element.

12. In an electron tube having an evacuated envelope and a photocathodehaving a photoemissive surface locating within said envelope. means forreducing the thermal generation of free electrons by said photocathode,comprising: a thermoelectric cooling element formed of materialexhibiting the Peltier effect and energizable to produce a cold surfaceand a hot surface;

means for mounting said cooling element adjacent to and exterior of saidenvelope with said cold surface in heat conductive relationship with theexterior surface of a selected segment of said envelope wall;

a thermal link within said envelope and having a first portion inintimate heat exchange relationship with the interior surface of saidselected segment of said envelope wall having a second portion ininitmate heat exchange relationship with said photocathode; and

means for energizing said thermoelectric cooling element whereby heat ispumped from said photocathode through said thermal link through saidselected segment of said envelope wall to the exterior of said envelope.

13. The device of claim 12, wherein said thermoelectric cooling elementcomprises at least one pair of coaxial loops, said loops beingoppositely energized whereby any magnetic field produced by currentflowing in one of said loops will be opposite to, and tend to cancel,any magnetic field produced by current flowing in the other of saidloops, thereby minimizing the effect of said magnetic fields on saidelectron tube.

14. The device of claim 13, further including heat sink means mounted inheat exchange relationship with the hot surface of said thermoelectriccooling element for dissipating heat pumped from said photocathode.

15. The device of claim 14, wherein said selected segment of saidenvelope is highly thermally conductive, said thermal link providing aheat conductive path between said photocathode and said selected segmentand the vacuum within said envelope thermally insulating saidphotocathode and said thermal link from the remainder of said electrontube, whereby the remainder of said tube is not cooled directly by saidthermoelectric cooling element.

16. The device of claim 15, wherein said selected segment is an annularband in the wall of said envelope, said thermoelectric cooling elementbeing annular and surrounding said tube with its interior circumferenceforming said cold surface and being in heat exchange relationship withsaid annular band.

17. The device of claim 12 wherein said selected segment is disc-shaped,said thermoelectric cooling element being generally disc-shaped andhaving a pair of spaced plates forming said hot and cold surfaces, saidcold surface plate being in heat exchange relationship with saiddisc-shaped segment.

18. The device of claim 17 wherein said thermoelectric cooling elementcomprises a plurality of thermocouples secured between said spacedplates, said thermocouples being arranged to minimize the production ofmagnetic fields within said electron tube.

1. IN A PHOTOEMISSIVE DEVICE, MEANS FOR REDUCING THE THERMAL EMISSION OFELECTRONS, COMPRISING AN EVACUATED ENVELOPE; A PHOTOEMISSIVE SURFACEWITHIN SAID ENVELOPE; A THERMAL LINK BETWEEN SAID SURFACE AND A PORTIONOF SAID ENVELOPE; AND A THERMOELECTRIC COOLING ELEMENT OUTSIDE SAIDENVELOPE AND IN HEAT EXCHANGE RELATIONSHIP WITH SAID PORTION OF SAIDENVELOPE, SAID COOLING ELEMENT BEING ENERGIZABLE TO PUMP HEAT FROM SAIDTHERMAL LINK AND THUS FROM SAID SURFACE IN ORDER TO COOL SAID SURFACE.2. The photoemissive device of claim 1, wherein said thermal linkcomprises a highly thermally conductive element in intimate heatexchange relationship with said photoemissive surface and with saidenvelope.
 3. The photoemissive device of claim 2, wherein at least saidportion of said envelope is of a highly thermally conductive material.4. The photoemissive device of claim 3, wherein said envelope isprimarily of glass, said portion of said envelope being metal.
 5. Thephotoemissive device of claim 4, wherein said photoemissive surfacecomprises a photocathode.
 6. The photoemissive device of claim 5,wherein said thermal link support said photocathode within saidenvelope.
 7. The photoemissive device of claim 5, wherein saidthermoelectric cooling element comprises a Peltier effect device andmeans for applying electric power to said device for energizationthereof.
 8. The photoemissive device of claim 7, wherein said thermallink is an electrical insulator.
 9. The photoemissive device of claim 8,further including a heat sink in thermal contact with said coolingelement for dissipating heat pumped from said photocathode.
 10. Thephotoemissive device of claim 9, wherein said cooling elemnt is in heatexchange relationship with only said portion of said envelope, wherebyonly said thermal link and said photocathode are cooled.
 11. Thephotoemissive device of claim 10 wherein said portion of said envelopecomprises thermally isolated section forming a portion of the wall ofsaid envelope, said thermal link providing a heaT conductive pathbetween said photocathode and said wall section, and the vacuum withinsaid envelope thermally insulating said photocathode and said thermallink from the remainder of said envelope, whereby the remainder of saidenvelope is not cooled by said cooled element.
 12. In an electron tubehaving an evacuated envelope and a photocathode having a photoemissivesurface locating within said envelope, means for reducing the thermalgeneration of free electrons by said photocathode, comprising: athermoelectric cooling element formed of material exhibiting the Peltiereffect and energizable to produce a cold surface and a hot surface;means for mounting said cooling element adjacent to and exterior of saidenvelope with said cold surface in heat conductive relationship with theexterior surface of a selected segment of said envelope wall; a thermallink within said envelope and having a first portion in intimate heatexchange relationship with the interior surface of said selected segmentof said envelope wall having a second portion in initmate heat exchangerelationship with said photocathode; and means for energizing saidthermoelectric cooling element whereby heat is pumped from saidphotocathode through said thermal link through said selected segment ofsaid envelope wall to the exterior of said envelope.
 13. The device ofclaim 12, wherein said thermoelectric cooling element comprises at leastone pair of coaxial loops, said loops being oppositely energized wherebyany magnetic field produced by current flowing in one of said loops willbe opposite to, and tend to cancel, any magnetic field produced bycurrent flowing in the other of said loops, thereby minimizing theeffect of said magnetic fields on said electron tube.
 14. The device ofclaim 13, further including heat sink means mounted in heat exchangerelationship with the hot surface of said thermoelectric cooling elementfor dissipating heat pumped from said photocathode.
 15. The device ofclaim 14, wherein said selected segment of said envelope is highlythermally conductive, said thermal link providing a heat conductive pathbetween said photocathode and said selected segment and the vacuumwithin said envelope thermally insulating said photocathode and saidthermal link from the remainder of said electron tube, whereby theremainder of said tube is not cooled directly by said thermoelectriccooling element.
 16. The device of claim 15, wherein said selectedsegment is an annular band in the wall of said envelope, saidthermoelectric cooling element being annular and surrounding said tubewith its interior circumference forming said cold surface and being inheat exchange relationship with said annular band.
 17. The device ofclaim 12 wherein said selected segment is disc-shaped, saidthermoelectric cooling element being generally disc-shaped and having apair of spaced plates forming said hot and cold surfaces, said coldsurface plate being in heat exchange relationship with said disc-shapedsegment.
 18. The device of claim 17 wherein said thermoelectric coolingelement comprises a plurality of thermocouples secured between saidspaced plates, said thermocouples being arranged to minimize theproduction of magnetic fields within said electron tube.